1. Field of the Invention
The present invention relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to correct nonuniformity in signal intensities, of image data acquired by imaging with using plural receive coils, occurring due to sensitivity distribution of each receive coil.
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstruct an image based on MR (magnetic resonance) signals generated due to the excitation.
As one of the conventional high-speed imaging technique in the field of magnetic resonance imaging, the technique to receive data by using the PAC (phased array coil) having multiple surface coils as receive coils is used. However, the spatial sensitivity nonuniformity of each surface coil used as a receive coil increases in using the PAC and it is imperative to perform the spatial sensitivity correction of a receive coil on data.
The conventional sensitivity corrections of a receive coil on image data include the method to use WBC image data that is acquired by using the WBC (whole body coil) as a receive coil (see, for example, Japanese Patent Application (Laid-Open disclosure) No. 2005-237702). The MRI apparatus includes normally the WBC for reception and transmission and a receive coil. Some kind or another reference image data is required to correct the influence of the sensitivity nonuniformity of a receive coil. Accordingly, image data acquired by the WBC has been generally used as reference image data for sensitivity correction conventionally.
FIG. 1 is a flowchart showing a procedure of the conventional sensitivity correction processing with using a WBC as a receive coil. The symbols each including S with a number in FIG. 1 indicate respective steps of the flowchart.
As shown in FIG. 1, the sensitivity map pre-scan for acquiring sensitivity map data indicating the sensitivity distribution of a receive coil is performed as a pre-scan in the step S1 prior to the main scan that is an imaging scan for acquiring image data. In the sensitivity map pre-scan, data is acquired under a same imaging condition by using the PAC and the WBC as a receive coil alternatively. Then, PAC image data and WBC image data are respectively generated by the image reconstruction processing of pieces data acquired by respectively using the PAC and the WBC.
WBC image data is acquired as image data corresponding to a large area enough for the FOV (field of view) of the object. Therefore, when coupling of the WBC is adequately reduced, characteristic of the WBC becomes nearly uniform. Consequently, WBC image data becomes reasonably uniform image data and can be used as reference image data. Meanwhile, PAC image data becomes nonuniform image data by the influence of nonuniform sensitivity distribution of the PAC. Therefore, the sensitivity distribution of the PAC can be estimated by dividing PAC image data by WBC image data.
For that purpose, basic data for estimating sensitivity map data of the PAC is generated by dividing PAC image data by WBC image data in the step S2. At this time, a part of data having subthreshold signal intensity is masked by the threshold processing on PAC image data and WBC image data.
Then in the step S3, sensitivity map data of the PAC in a required spatial region is generated from the basic data. Specifically, pieces of data in regions where the division result can not be calculated are interpolated by interpolation and extrapolation.
Then in the step S4, data is acquired by the imaging scan for acquiring image data and image data is generated by the image reconstruction processing of the acquired data. Then, the sensitivity correction processing is performed on the image data by using the sensitivity map data of the PAC. That is, absolute values of the signal intensities of the image data are corrected based on the sensitivity map data. This allows nonuniformity of the image data to be corrected.
The technique to estimate sensitivity map data only from PAC image data has been devised as another sensitivity correction technique (see, for example, Japanese Patent Application (Laid-Open disclosure) No. 2005-237703). This technique easily estimates sensitivity map data having a rough accuracy by performing processing such as threshold processing, region reduction processing, interpolation processing, extrapolation processing and smoothing on PAC image data having a low contrast.
However, the conventional sensitivity correction technique with using the WBC might not correct nonuniformity of image data acquired by an imaging scan adequately.
FIG. 2 is a diagram showing an image of which nonuniformity due to sensitivity unevenness does not corrected adequately in spite of the conventional sensitivity correction with using the WBC. FIG. 3 is a diagram showing a profile of pixel values on respective pixel positions on a ROI (region of interest) which is represented with a line in an image, corrected under the conventional sensitivity correction method, shown in FIG. 2.
FIG. 2 shows a longitudinal relaxation (T1) weighted image of the head acquired by the conventional sensitivity correction by using WBC image data. Approximately 20% of sensitivity irregularity occurs on either side of the brain in the example shown in FIG. 2. For example, the pixel values of the part on the right side is reduced by approximately 20% by correction.
This insufficient sensitivity correction is due to nonuniformity of WBC image data acquired by the sensitivity map pre-scan. Nonuniformity of WBC image data is attributed in part to inter-coil coupling between the WBC and the PAC. That is, due to causes such as inter-coil coupling between the WBC and the PAC, sensitivity irregularity might be generated on reference WBC image data itself and also on estimated sensitivity map data. Sensitivity irregularity remains on image data after sensitivity correction when the sensitivity correction is performed by using sensitivity map data having sensitivity irregularity.
FIG. 4 shows conventional PAC image data acquired for generating sensitivity map data. FIG. 5 is a diagram showing a profile of pixel values on respective pixel positions on a linear ROI in the conventional PAC image data for generating sensitivity map data shown in FIG. 4. FIG. 6 shows conventional WBC image data acquired for generating sensitivity map data. FIG. 7 is a diagram showing a profile of pixel values on respective pixel positions on a linear ROI in the conventional WBC image data for generating sensitivity map data shown in FIG. 6.
FIG. 4 and FIG. 6 show examples of typical PAC image data and WBC image data acquired for estimating sensitivity map data of the PAC, respectively. As shown in FIG. 4 and FIG. 5, PAC image data acquired by the sensitivity map pre-scan has nonuniformity according to the characteristic of the PAC that is a receive coil. Basically, sensitivity map data is estimated based on data derived by dividing the PAC image data by the WBC image data (the PAC image data/the WBC image data). Therefore, it is important that the WBC image data becomes flat to some extent without sensitivity irregularity.
However, as shown in FIG. 6 and FIG. 7, WBC image data might not become adequately uniform. In the example of WBC image data shown in FIG. 6 and FIG. 7, a difference in the pixel value between the right and left sides is found. When sensitivity map data for sensitivity correction is calculated by using such nonuniform WBC image data as reference image data, a correction of a T1 weighted image that is a target of the sensitivity correction becomes actually insufficient.
Moreover, in a case where uniformity of the sensitivity distribution is low due to a dielectric artifact in a MRI apparatus that generates a high magnetic field, in a case where the sensitivity distribution of the PAC is nonuniform and under various conditions, adequate sensitivity correction might be difficult.